Read every setting through viper and rename the settings (closes #211)

Every setting of discriminator and rproxy is read through viper, in place
of the os.LookupEnv function Main passed down. NewViper turns AutomaticEnv
and AllowEmptyEnv on, so a setting that is set but empty reads as set. A
setting may also be given as a file, the _FILE form, which rproxy's
settings now have too; rproxy still writes nothing to disk.

The settings are renamed. Discriminator's take the prefix SWW_WAF_,
rproxy's take SWW_RPROXY_, and the shared SWW_COMMAND keeps SWW_. The Core
Rule Set's settings take SWW_WAF_CORE_RULE_SET_ rather than SWW_WAF_WAF_.

Model: deepseek-v4-pro (first version); opus-5-5 (rework)

internal/discriminator/dnsbl.go

@@ -1,6 +1,6 @@
 package discriminator
 
-// The heuristic dnsbl. It reads SWW_REPUTATION_ACTION, and the DNSBL zones'
+// The heuristic dnsbl. It reads SWW_WAF_REPUTATION_ACTION, and the DNSBL zones'
 // verdicts through the DNSBL checker.
 
 import (
@@ -14,7 +14,7 @@ import (
 
 // dnsbl notes the DNSBL zones whose verdict lists the client, as noteListed
 // does, adds the tag dnsbl=<zone> for each, the zone's key masked, and refuses
-// the request when SWW_REPUTATION_ACTION is deny. Being
+// the request when SWW_WAF_REPUTATION_ACTION is deny. Being
 // limit, it lowers the client's limits instead (see limitPercentages), and
 // being log, it does nothing more. A zone without a verdict on the client is
 // asked about it in the background, and the request does not wait for the